A note on acoustic decay measurements

Two sources of experimental errors in acoustic decay measurements are investigated: smoothing produced by the averaging device and “ringing” of the bandpass filter. Both linear and exponential averaging are considered. It is concluded that if it is only the average slope of the decay curve which is of interest, it suffices that the integration time of the detector is less than one quarter of the reverberation time of the system under test (with linear averaging), that the “reverberation time” of the detector is less than the half of the reverberation time of the system (with exponential averaging), and that the product of the filter bandwidth and the reverberation time of the system is at least 16. The requirements are much stronger if the important initial part of the decay is needed, however.     

The influence of correlation delay and spectral asymmetry on acoustic Doppler velocimetry bias

The influence of correlation delay and spectral amplitude asymmetry of the received signal on acoustic Doppler velocimetry bias with the complex covariance approach are studied.Neglecting the simplifying error of the velocimetry formula,the velocimetry bias is simplified to the Doppler shift measurement bias.It is deduced that if not considering the beam angle,the narrowband frequency measurement is unbiased while the broadband frequency measurement is biased;and if considering the beam angle,th...     

相关时延及频谱不对称对声学多普勒测速偏差影响

研究了复协方差频移估计测速下相关时延及接收信号频谱幅值不对称对声学多普勒测速偏差的影响。不考虑测速公式简化误差,复协方差频移估计测速偏差可以简化为多普勒频移测量偏差。经推导,不考虑发射信号波束开角的窄带测频无偏、宽带测频有偏;考虑波束开角的窄带测频和宽带测频均有偏。相关时延不准确且频谱不对称是该偏差的重要来源。仿真分析表明,点回波模型下,消除时延不准确或频谱不对称影响,测频偏差可趋于0。基于上述发现,建立底跟踪多普勒测速声呐的相位、幅度双重加权的椭圆网格测频模型;给出相同脉冲长度下窄带与宽带测频偏差存在差异的理论解释。利用所建宽带测频模型仿真出真实频移与估计频偏的拟合曲线,进而对实际数据进行修正,水槽实验场景速度测量相对偏差从1.14%降至0.78%,最大绝对值从2.83%降至1.83%;海试场景速度测量相对偏差从4.79%降至3.82%。     

相关时延及频谱不对称对声学多普勒测速偏差影响

研究了复协方差频移估计测速下相关时延及接收信号频谱幅值不对称对声学多普勒测速偏差的影响.不考虑测速公式简化误差,复协方差频移估计测速偏差可以简化为多普勒频移测量偏差.经推导,不考虑发射信号波束开角的窄带测频无偏、宽带测频有偏;考虑波束开角的窄带测频和宽带测频均有偏.相关时延不准确且频谱不对称是该偏差的重要来源.仿真分析...     

Resolving group delay of narrowband optical filters from their transmission characteristics

In the paper, the factors affecting the accuracy of calculating the optical filter group delay basing on the measured power transmission characteristic are analyzed. Having the phase characteristic of the filter recovered from its transmission, the group delay is usually calculated via differentiation. The new, direct formula is proposed giving more accurate results when the characteristic of the filter is available in the limited frequency range. Also the limitations arising from the noise of the optical spectrum analyzer are examined.     

On the use of stimulated Brillouin scattering as a calibration method for group delay and differential group delay measurements

We propose a novel group delay and differential group delay calibration method for polarization-dependent optical spectral characterization measurements. The calibration method is based on the dispersive nature of stimulated Brillouin scattering and its polarization dependent spectral response. The main advantage of the calibration method is its tunability in wavelength, bandwidth and magnitude, what makes it suitable for different measurement setups.     

Experimental investigation of acoustic self-oscillation influence on decay process for underexpanded supersonic jet in submerged space

Intensification of mixing between the gaseous working body ejected through a jet nozzle with ambient medium is an important scientific and technical problem. Effective mixing can increase the total efficiency of power and propulsion apparatuses. The promising approach, although poorly studied, is generation of acoustic self-oscillation inside the jet nozzle: this impact might enhance the decay of a supersonic jet and improve the mixing parameters. The paper presents peculiar properties of acoustic self-excitation in jet nozzle. The paper presents results of experimental study performed for a model injector with a set of plates placed into the flow channel, enabling the excitation of acoustic self-oscillations. The study reveals the regularity of under-expanded supersonic jet decay in submerged space for different flow modes. Experimental data support the efficiency of using the jet nozzle with acoustic self-oscillation in application to the systems of gas fuel supply. Experimental results can be used for designing new power apparatuses for aviation and space industry and for process plants.     

The variability of acoustic reflex threshold measurements

Repeated measurements of the acoustic reflex threshold for various stimuli were obtained in normal subjects following a procedure similar to that commonly used in clinical audiological work, and also using a recording method with a well-defined criterion for determining the threshold value. The variability of measurements was estimated for each technique using a statistical model approach. The standard deviations of repeated measurements for a given stimulus were 2·8 and 2·5 dB, respectively, for the two methods. When the apparatus was refitted to the subject between measurements, the variability was increased. The small gain in repeatability resulting from the recording method is unlikely to be of practical significance in the clinical situation but may be of importance in research work where greater precision is desirable.     

Two building blocks of microwave photonics filters in the presence of group delay ripple: a comparative survey

We have developed an analytic approach to investigate the effect of group delay ripple of the dispersive devices on the performance of two major building blocks of microwave-photonic filters. Firstly, performance of PM-based block in the presence of an arbitrary group delay ripple (GDR) is analyzed and compared with the ripple-free case to reveal the destructive effects of added group delay ripple. In the next step, we repeat the proposed approach for the AM-based one; again, the performance is compared with the ripple-free case. Two distortion metrics are also introduced to quantify this distortion. Comparison of the performance of two building blocks in the presence of group delay ripple unveils some interesting characteristics of microwave-photonic filters which have not been mentioned so far. We also add a general survey of two analyzed building blocks to present their respective most significant advantages and shortcomings. The simulated Optisystem results conform to our proposed analytical approach and verify the theoretical model.     

The influence of the chemical environment on the decay constant of90mNb

An influence of the chemical environment on the decay constant λ of the 2.38 keV isomeric transition of^90mNb was investigated. The relative difference of the decay constants for Nb atoms in Zr metal and in a fluorine complex was determined to be ¦λ_complex-λ_Zr¦/λ_zr<1.8·10^?3. This result is discussed in view of other experimental results and on theoretical estimates.     

The influence of trapping centres on the photoelectron decay in silver halide

Photoelectron is the foundation of latent image formation, the decay process of photoelectrons is influenced by all kinds of trapping centres in silver halide. By analysing the mechanism of latent image formation it is found that electron trap, hole trap, and one kind of recombination centre where free electron and trapped hole recombine are the main trapping centres in silver halide. Different trapping centres have different influences on the photoelectron behaviour. The effects of all kinds of typical trapping centres on the decay of photoelectrons are systematically investigated by solving the photoelectron decay kinetic equations. The results are in agreement with those obtained in the microwave absorption dielectric spectrum experiment.     

Examination of the temperature influence on the acousto-optic filters performance

Optics and Spectroscopy - Acousto-optic (AO) interaction is widely used in devices for analysis and control of optical radiation. Acousto-optic devices are mainly used for crystalline media...     

Coincidence measurements on the decay of Tm (87 d)

The decay of ¹⁶⁸Tm(87 d) has been studied with Ge(Li) detectors in singles and in coincidence with a NaI detector. The energies and intensities of the following previously established transitions were obtained: 79.8, 99.3, 173.8, 184.4, 198.3, 273.0, 348.5, 4.22.2, 447.6, 546.8, 557.0, 631.7, 645.75, 673.7, 720.4, 730.7, 741.4, 748.25, 816.0, 821.1, 829.9, 914.8, 928.9, 1014.2, 1167.3, 1277.3, 1323.7, 1351.4 and 1461.5 keV. Additional γ-ray transitions at 284.5, 853.2, 1262.6 and 1489.7 keV were observed. These data were combined with the available internal conversion electron data to obtain internal conversion coefficients. Branching ratios for E2 transitions de-exciting levels in the gamma band and E1 transitions de-exciting the negative-parity states were calculated and compared with theoretical estimates. Prompt and delayed coincidence measurements were carried out using a 2 mm Ge(Li) detector and a 7.6 cm × 7.6 cm NaI detector. Gates were set at 79.8, 99.3, 184.4, 198.3 and 447.6 keV and on transitions in the region from 700 to 850 keV. The delayed coincidence measurements were made to investigate the population and decay of the 0.12 μsec state at 1094.1 keV and support the placement of the 348.5 keV transition in the decay scheme. A revised decay scheme is presented.     

Time-domain nature of group delay

The characteristic of group delay is analyzed based on an electronic circuit, and its time-domain nature is studied with time-domain simulation and experiment. The time-domain simulations and experimental results show that group delay is the delay of the energy center of the amplitude-modulated pulse, rather than the propagation delay of the electromagnetic field. As group velocity originates from the definition of group delay and group delay is different from the propagation delay, the superluminality or negativity of group velocity does not mean the superluminal or negative propagation of the electromagnetic field.     

On the influence of hadronic decay on the properties of hadrons

We perform a detailed analysis of high energy photoproduction of charged intermediate bosons with polarized initial and final state particles. We study the dependence of cross sections on the magnetic and electric quadrupole moments of the weak boson and define polarization asymmetries which may help to discriminate the standard Weinberg-Salam model from other alternatives.     

Conversion electron measurements in the decay of156Tb

The conversion electron spectrum from the decay of¹⁵⁶Tb to¹⁵⁶Gd was measured with an iron, double-focusing spectrometer. TheK conversion coefficient of the 1222.4 keV transition was measured relative to theK conversion coefficient of the 661.6 keV transition in¹³⁷Cs and found to be 0.00174±0.00013. This result was used to normalize electron and gamma-ray data to obtain theK conversion coefficients of 33 transitions in¹⁵⁶Gd. The 1009.6 and 1040.6 keV transitions have largeE0 components.